In relatively large buildings designed to withstand earthquakes, the expansion joints are virtually always designed to endure excursions of the building members at the joints of more than four inches and may be designed for excursions of up to 20 inches or more. Conventionally, the expansion gap is bridged by an elongated cover, one side of which is supported on a frame affixed to a building member on one side of the gap and the other side of which is supported on a frame affixed to a building member on the other side of the gap. Ordinarily, the cover of an expansion joint cover is fabricated from a flat center plate of aluminum plate stock and an adapter or nosing along each side. The ability of the center plate to support loads is, of course, a function of the thickness of the plate material. The wider the expansion gap at the maximum excursion of the building members away from each other in an earthquake, the thicker the cover plate must be for a given load-carrying ability. Aluminum plates of a thickness of, say, ¼ inch or more, are required for relatively wide expansion gaps.
Aluminum plate stock is relatively costly. Fabrication of the center plates from plate stock also involves a fair amount of waste. Usually, the adapters or nosings are welded to the center plate. The cost of a cover for a seismic expansion joint cover can be appreciable, due to the high cost of the plate stock, the waste, and the labor and capital costs for welding the adapters or nosings to the center plate.